French patent 2 728 809 discloses a pump having a pushbutton mounted to move on a support which is secured to the receptacle containing the substance to be dispensed, the pushbutton comprising a circularly-cylindrical central duct having radial openings at its bottom end, the support defining around said duct an annular pump chamber of variable volume. A membrane made out of an elastomer is mounted on the support. The membrane has a circulary-symmetrical central portion in the form of a sleeve that is open at its top end and closed at its bottom end. The central duct of the pushbutton is inserted in the membrane until it bears against the end wall of the sleeve.
The membrane thus constitutes a resilient return member enabling the pushbutton to be brought back to its initial position after a quantity of substance has been dispensed.
In addition, during the return movement of the pushbutton the membrane isolates the pump chamber by pressing against the central duct, thus serving to prevent air from penetrating therein.
Such a pump has the advantage of comprising a small number of parts only and therefore of being relatively inexpensive to make.
Nevertheless, that prior art pump does not give entire satisfaction; the Applicant company has observed that the pushbutton tends to jam and/or that the quantity of substance tends to be irregular and poorly dispensed.